The present invention relates generally to magnetic random access memory devices, and more particularly, a novel structure for a magnetic random access memory device that is programmed by spin torque transfer, and a method for making the structure.
Conventional non-volatile magnetic random access memory (MRAM) devices are operated by using cross-point magnetic field switching. A field is generated by a current through bit lines arranged above and below the device. A ferromagnetic free layer in the device, serving as the writable plate, has a coercivity field that is smaller than the magnetic field generated by the bit lines. As a result, the magnetization direction of the ferromagnetic free layer is altered to align with the magnetic field direction. However, this operation method limits the ability to scale down the MRAM device because a large current is required. When a plurality of devices are arranged in an MRAM array, the operation additionally has a problem of write disturbance because the magnetic field of one device also affects the ferromagnetic free layer of neighboring MRAM devices.
On the other hand, an MRAM device may also use a method called spin-torque transfer for write operations. The operation depends on a current density flowing through a magnetic tunnel junction (MTJ) stack rather than current amplitude. The MTJ stack is composed essentially of a ferromagnetic free layer and a reference layer having a fixed magnetization direction. Electrons pass through the reference layer and are spin polarized. As the electrons flow from the reference layer through the ferromagnetic free layer, the electrons gradually change the magnetization direction of the ferromagnetic free layer based on a tuning of the precessing and damping terms of the Landau-Lifshitz-Gilbert (LLG) equation. Additionally, by using a spin-torque transfer current through the MTJ stack, the cell being programmed may be written without disturbance. This enables self-writing, i.e., no additional bit line is required for writing contribution. As a result, the MRAM devices may be scaled down even further.
The MTJ stack may also utilize a second reference layer. The second layer has a magnetization direction opposite to that of the first reference layer. The subsequent magnetization direction of the free layer is thus determined by the direction of the flow of electrons through the memory device during the write operation. For example, for a current flowing through the first reference layer to the second reference layer, the magnetization direction of the free layer is aligned to that of the first reference magnet.
The spin torque transfer current density required strongly depends on the size of the MTJ stack. However, as the MTJ stack becomes smaller, the device suffers from memory information loss caused by superparamagnetism. Since high writing current densities are required to effect the change in the free layer, thermal energy becomes substantial enough to cause the atomic magnetic moments in the material to fluctuate randomly. This phenomenon contributes not only to destabilization of the ferromagnetic free layer, but to destabilization of the reference magnets as well. The problem of write disturbance in the MRAM arrays also persists. It is therefore desirable to use an MRAM device which requires a lower writing current density and which is capable of maintaining a stable magnetic state in both the device itself and the array.